Views: 0 Author: Site Editor Publish Time: 2023-04-14 Origin: Site
Abstract: The effect of hydroxypropyl methylcellulose (HPMC) on the dispersion of carbon fibers in cement paste was comprehensively evaluated by the coefficient of variation of carbon fiber in fresh cement paste, the resistivity of hardened cement paste, and SEM analysis tests. Influence. The test found that: HPMC can significantly improve the dispersion of fibers, and the dispersion effect is better when the dosage is 0.2% to 0.4%. The analysis suggests that the mechanism lies in the formation of hydrogen bonds between the polar hydroxyl groups in its molecular structure and the polar hydroxyl groups or carbonyl groups on the surface of carbon fibers and water molecules, which enhances the hydrophilicity and wettability of the short carbon fiber surface and improves The dispersibility of short carbon fibers is improved; however, the larger the dosage, the more cohesiveness of the system will affect the dispersibility, and the resistivity will increase due to the introduction of more air bubbles.
Key words:carbon fiber; dispersion; hydroxypropyl methylcellulose; mechanism
The dispersion state of carbon fiber in concrete is a key issue in the preparation and application of carbon fiber concrete, which has an important impact on its electrical conductivity, electric-force and force-electric effects. Scholars at home and abroad have carried out a lot of research work on the dispersion of carbon fibers. D. D． L． Chung first used methyl cellulose (MC) as a dispersant to improve fiber dispersion. In addition, she also proposed two methods for surface modification of carbon fibers: one is to soak carbon fibers in a strong oxidant solution or treat them in ozone to form hydrophilic oxygen-containing functional groups on the surface; the other method It is to soak the carbon fiber in the silane coupling agent solution to form a silane coating on the surface of the fiber to improve the hydrophilicity. Sun Hui, Sun Mingqing and others found that adding carboxymethylcellulose sodium (CMC) and silica fume to the cement slurry can significantly improve the dispersion of carbon fibers. Wang Chuang et al. used methyl cellulose (MC), sodium carboxymethyl cellulose (CMC), and hydroxyethyl cellulose (HEC) as three commonly used dispersants and found that the dispersant effect of the dispersant on short carbon fibers was HEC>CMC> MC.
Hydroxypropylmethylcellulose (HPMC), a derivative of methylcellulose, can be used as a tackifier and dispersant in concrete, but it has rarely been involved in previous studies on carbon fiber dispersion. In this paper, the influence of HPMC on the dispersion of carbon fibers was studied by means of fresh slurry method, hardened cement slurry resistance test method and cross-sectional SEM test, and its mechanism was analyzed.
2.1 Raw materials
5mm PAN-based carbon fiber, (Shanghai Xinka Carbon Technology Co., Ltd.); hydroxypropyl methylcellulose (KIMA CHEMICAL CO., LTD); 42.5P. O cement (Huaxin Cement Factory); tributyl phosphate (Tianjin Hengwu Technology Industry and Trade Co., Ltd.) and polycarboxylate superplasticizer (Wuhan Haoyuan Chemical Building Materials Co., Ltd.).
2.2 Test method
2.2.1 Predispersion of short carbon fibers
Heat the mixing water to 90°C, add the weighed dispersant into the water and stir until completely dissolved, cool to 40°C, then add defoamer and carbon fiber in turn, stir with a glass rod for 2 minutes, then cool down for use.
2.2.2 Measuring method of fiber dispersion
Add the weighed cement and water reducer in turn into the pre-dispersed fiber-water mixture and mix them evenly. Take out 6 samples with a mass of 60g each from different positions of the freshly mixed slurry, carefully sieve the cement with water and a 60-mesh fine sieve, then dry it, weigh the mass of the residual carbon fiber, and measure it by the variation coefficient and difference rate Fiber dispersion.
2.2.3 Resistivity test method
The carbon fiber cement slurry prepared above is used to form a prism specimen of 4cm×4cm×16cm, and the stainless steel sheet electrode is embedded in advance. After 28 days of standard maintenance, the resistivity test is carried out by the four-electrode method, and the fiber is reflected from the side according to the size and variation coefficient of the resistivity. of dispersion. Calculated according to the mass percentage of cement, the contents of other components are: water reducer 0.5%; defoamer 0.03%; HPMC content is 0, 0.2%, 0.4% and 0.0%. 6%; water-cement ratio 0.30. The carbon fiber content is 0.5% of the cement slurry volume. The difference between the resistivity and the average resistivity (resistance difference rate) can reflect the uniformity of fiber distribution.
3. Test results and analysis
3.1 Dispersion state of fibers in aqueous solution
Under the premise of the same dispersion process, the carbon fiber is dispersed in the aqueous solution mixed with HPMC and without HPMC. When HPMC is not added, the carbon fiber is unevenly distributed in the water. In some areas, the fibers are bonded into bundles and entangled with each other. Poor, there is obvious layering and segregation phenomenon in the aqueous solution; after adding HPMC, the dispersion of the fiber in the water is relatively uniform, the carbon fiber is suspended in the water, showing good compatibility, and the flow of the carbon fiber and the aqueous solution when pouring the aqueous solution It is uniform, and there is no separation of water and fibers when HPMC is not added.
3.2 The effect of HPMC content on the coefficient of variation and resistivity of carbon fiber mass in cement paste
Analyzing the above experimental results, it can be found that: (1) From the influence of the amount of HPMC on the coefficient of variation and difference rate, it can be seen that after adding 0.2% HPMC, the coefficient of variation drops from 14.3 to 2.0, a drop of 86%. ; As the content increased to 0.4% and 0.6%, the coefficient of variation decreased by 88% and 70% respectively compared with the time without mixing; it shows that the addition of HPMC significantly reduces the coefficient of variation and greatly improves the fiber in cement paste. (2) In the influence of HPMC dosage on the coefficient of variation and difference rate, the coefficient of variation appears to become smaller and then larger with the increase of dosage, which is mainly due to the increase of HPMC dosage. The viscosity of the solution also increases. When the viscosity exceeds a certain value, the stirring of the fiber suspension becomes more difficult, and it is not easy to stir evenly. It can also be seen from the change trend of the difference rate that with the increase of the HPMC content, the difference first becomes smaller and then becomes larger, and reaches the maximum when the content is 0.6%, which verifies the coefficient of variation from the side. Therefore, there is an appropriate range for the amount of HPMC, and it is considered to be better between 0.2% and 0.4% in the test; (3) In the influence of the amount of HPMC on the resistance value, adding HPMC can effectively reduce the resistivity of the sample. After adding 0.2%, the resistivity is reduced from 43.70Ω·cm to 16.73Ω·cm, with a drop rate of 61.7%. It can be found that the trend between the conductivity and the variation rate is basically the same, and there is a minimum value between 0.2% and 0.3%; (4) The effect of HPMC content on the resistance value In the effect, the resistivity rebounded when the dosage of HPMC was increased, from 16.73Ω·cm with 0.2% dosage to 38.43Ω·cm with 0.6% dosage. The overall resistivity presents a trend of decreasing first and then increasing.
3.3 The relationship between the content of HPMC and commonly used methylcellulose derivatives and the coefficient of variation
Use the method of 2.2.2 to test common cellulose derivatives sodium carboxymethylcellulose (CMC) and hydroxyethylcellulose (HEC).
Among the variation coefficients of different dispersants at the same dosage, when the dosage of dispersant is 0.2%, the variation coefficient of carbon fiber mixed with HPMC is the smallest, only 1.7%, while the variation coefficients of CMC and HEC are both More than 6%, the coefficient of variation HPMC<HEC<CMC; when the dosage of dispersant is 0.4%, the coefficient of variation of adding HPMC has little change compared with the dosage of 0.2%, and the ratio of adding CMC and HEC Compared with the dosage of 0.2%, the coefficient of variation has decreased to a certain extent, and the coefficient of variation is HPMC<CMC<HEC; when the dosage of dispersant is 0.6%, the coefficient of variation is HEC<CMC<HPMC. From the relationship between the variation coefficient of different dispersants and the dosage, it can be seen that after the addition of methyl cellulose derivatives CMC and HEC, when the dosage increases from 0.2% to 0.6%, the cement slurry The coefficient of variation of carbon fiber shows a decreasing trend with the increase of the dosage. The experiments of Zhang Hui, Sun Mingqing, etc. also proved that the dispersion of the fiber can reach the best effect when the dosage of CMC is 0.8%. When the fiber has reached the best dispersion effect, its dispersion effect is better than that of CMC and HEC with a content of 0.6%. Comprehensive analysis of the coefficient of variation of different dispersants at the same dosage and the relationship between the coefficient of variation and dosage of different dispersants can be concluded: HPMC is a very good fiber dispersant, which can reach the commonly used Dispersion effects of CMC and HEC.
3.4 SEM test analysis of carbon fiber dispersion
SEM test was carried out after hardening 28d slurry samples.
As shown in the SEM image of the hardened cement paste without HPMC and the SEM image of the hardened cement paste with HPMC, the dispersion of carbon fibers in the hardened cement paste without HPMC is poor, and the fiber distribution in some areas is very dense and intertwined. The phenomenon is very obvious, and there is no fiber distribution area in some areas; after the addition of HPMC, the dispersion of the fibers is significantly improved, and the relative spacing is stable, indicating that the addition of HPMC can improve the dispersion of the fibers and promote the fibers in the same amount. Better overlapping into a complete conductive network, avoiding or reducing the local clustering of fibers, and the absence of local fibers resulting in interruption of the conductive network. At the same time, the above phenomenon also explains to a certain extent the reason for the large correlation between the hardening resistance conductivity and the variation rate.
3.5 Mechanism analysis of HPMC improving fiber dispersion
PAN-based carbon fiber has a stable non-polar ring structure, which is incompatible with polar water molecules, making the carbon fiber poorly dispersible in water. This problem can be solved by methods such as chemical reaction to reform polar bonds and adsorption of polar bonds. Among them, the surface modification of carbon fiber belongs to the former, and the dispersant belongs to the latter.
HPMC, CMC, HEC and other cellulose derivatives all contain hydrophilic group hydroxyl group and hydrophobic group hydrocarbon group in the molecular unit structure, but the difference lies in the molecular chain structure and length of the group. HPMC has more than CMC and HEC. Polar groups, longer molecular chains. Polar groups such as hydroxyl or carbonyl form hydrogen bonds with polar water molecules, and at the same time form monomolecular films and spherical micelles on the surface and inside of the liquid to reduce the area between water and air, so that the surface tension of the solution is reduced sharply, thereby Increase the hydrophilicity and wettability of carbon fiber, improve its dispersibility, and form a colloidal dispersion system. Since HPMC has more polar groups, it has a better dispersion effect than CMC and HEC at the same dosage. At the same time, because the non-polar fiber surface is very easy to absorb the non-polar groups in the dispersant, a film with the non-polar groups facing inward and the polar groups facing outward is formed on the fiber surface, reducing the distance between the fiber and the aqueous solution. Surface tension further improves the ability of fibers to disperse freely in aqueous solution. Also due to the larger molecular weight and longer branched chains in HPMC, HPMC is easier to process than CMC and HEC at the same lower dosage.
The conditions for the formation of the hydrophilic layer are met, and the fibers are more fully dispersed. However, if the content is too high, the viscosity of the solution is too high, which affects the fluidity of the dispersion and hinders the dispersion of carbon fibers.
HPMC also greatly reduces the surface tension between air and water, and introduces more air bubbles into the aqueous solution. Although it also has a certain effect on preventing carbon fibers from agglomerating again, it has a negative impact on electrical conductivity and mechanical properties, so it is necessary to add defoaming agent to reduce the formation of air bubbles. It can be seen from the comparison that after the addition of HPMC, the number of pores in the cement stone increases significantly, and there are many pores with a diameter greater than 100/um, despite the use of a defoamer. Too many and too large pores will inevitably destroy the integrity of the conductive network in the entire cement paste. Therefore, this is also one of the reasons why the conductivity of cement paste decreases first and then increases with the increase of HPMC content.
The addition of HPMC is very effective in improving the dispersion state of fibers in cement paste; however, there is an appropriate range for the amount of HPMC, and the test believes that it is better between 0.2% and 0.4%. When the dosage reaches 0.6%, due to the high cohesion of the system, the dispersion of carbon fibers will be affected; at the same time, due to the introduction of more air bubbles, the integrity of the conductive network of the cement paste will be affected, and the resistivity will be increased. . Since HPMC has a larger molecular weight, longer molecular chain and more polar groups, the effect is better than that of CMC and HEC in the range of lower dosage.